The present invention relates to solid state electrochemical devices and, more particularly, to solid state electrochemical devices in which the electrolyte is a polymeric network interpenetrated by an ionically conducting liquid phase.
Traditional batteries, employing aqueous solutions as the electrolytes, have given way to electrochemical devices, such as batteries and capacitors, which have a solid electrolyte. Unlike their aqueous electrolyte counterparts, the solid electrolyte devices offer the advantages of thermal stability, absence of corrosion of the electrodes, and the ability to be manufactured in thin layers.
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art. These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte, i.e., liquid batteries, including improved safety features.
The solid electrolyte is interposed between the cathode and the anode. To date, the solid electrolytes have been constructed from either inorganic or organic matrices including a suitable inorganic ion salt. The inorganic matrix may be non-polymeric e.g, .beta.-alumina, silver oxide, lithium iodide, etc.! or polymeric e.g., inorganic (polyphosphazine) polymers! whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix.
One problem which research efforts have attempted to overcome in the design of solid state cells from a polymeric matrix is the poor conductivity of polymeric electrolytes at room temperature and below. In many cases, the cells which have been designed to date are used at elevated temperatures due to the low conductivity of the electrolyte at ambient temperatures.
In addition to providing a high ionic conductivity, it is important that a polymeric electrolyte also provide good mechanical strength. Unfortunately, there is a tendency for these two properties to oppose one another. Attempts to increase conductivity usually involve a reduction in molecular weight and result in fluid or mechanically weak films. Techniques, such as crosslinking, increase film strength but generally result in a loss in conductivity.
The problem of striking a suitable balance between these two mutually exclusive properties has been solved to some extent by providing a solid polymeric electrolyte which is a two phase interpenetrated network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionic conducting phase comprising a metal salt and a complexing liquid polymer such as liquid polyethylene oxide complexed with a lithium salt, as set forth in U.S. Pat. No. 4,654,279. As explained therein, the mechanically supporting phase forms a matrix which supports an interpenetrating ionically conducting liquid polymer phase which provides continuous paths of high conductivity throughout the matrix. Representative examples of the mechanically supporting phase described in U.S. Pat. No. 4,654,279 are epoxies, polyurethanes, polyacrylates, polymethacrylates, polystyrenes and polyacrylonitriles.
The solvent (plasticizer) is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte are recognized by those skilled in the art be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate,.gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglime, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte is typically formed in one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (i.e., the glyme of Formula I and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
The development of the solid polymeric electrolyte including the two phase interpenetrated network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionic conducting phase comprising a metal salt of a complexing liquid polymer overcame to a significant extent the problem of striking a balance between good mechanical strength on the one hand and good conductivity on the other hand.
One particularly preferred solid electrolyte battery, including a crosslinked polymeric phase and an ionic conducting phase, is made employing lithium as the anode. In particular, lithium has been of interest due to its low density and highly electropositive nature. A typical cell will incorporate, for example, a lithium or lithium based anode and a cathode containing a vanadium oxide compounds, V.sub.6 O.sub.13 as the active material. The lithium anode may be a metal foil. The electrolyte layer consists of a polymer such as polyethylene oxide and a lithium salt. The cathode structure consists of a composite material containing the active cathode material V.sub.6 O.sub.13, a polymer electrolyte material, and carbon in the form of acetylene black. These batteries have been found to be beneficial in terms of ease of construction, ruggedness, interfacial properties, open circuit voltage, energy density, and rechargeability.
Despite its otherwise superior suitability for use in solid electrolyte batteries, the present inventor has found that in the case of highly reactive anodic metals such as lithium, the anode can actually react with the solid polymeric electrolytes. Such a reaction causes the formation of a corrosion layer between the anode and the electrolyte. Because such corrosion layer has a high resistance, it causes a significant decrease in the energy content and the peak current of the battery and thus, seriously undermines the operability of the battery.